One of the basic requirements in an automated control system is to coordinate the activity of multiple independent mechanical systems in real time. Often, the independent mechanical systems must be orchestrated in such a maimer that the motions of several of the systems are required to culminate at the same time, even though each system is driven through different methods, speeds and/or gear ratios. This is difficult to accomplish in a system where one or more of the drive component systems operate at variable speeds.
An image transfer section of a printer is one example of multiple mechanical systems that must be controlled in a precise manner in order to assure accurate transfer of the image onto the card. Typically an image transfer section includes a pair of applicator rollers forming a roller nip, with one of the applicator rollers being heated. A plastic card and a receptor web pass through the nip to laminate a portion of the receptor web onto the card. The portion of the receptor web that is laminated onto the card has an image printed thereon, and therefore the printed image is transferred from the receptor web onto the plastic card. The applicator rollers are each driven in opposite directions through a suitable connection to a drive motor, and one of the applicator rollers is moveable toward and away from the other applicator roller, thereby varying the gap between the applicator rollers. The plastic card is driven into the nip of the applicator rollers by a pair of drive rollers which are driven in opposite directions by a drive motor, and the receptor web is pulled through the nip of the applicator rollers by a web take-up roll which is driven by another drive motor. In this system, an image is first printed onto the receptor web in a printing station, and the image is then transferred onto the card by the applicator rollers.
One of the primary problems associated with an image transfer section is the requirement to register the card, the applicator rollers and the receptor web in the horizontal direction (X-axis direction) and to register the card and web in the vertical direction (Y-axis direction), in order to assure proper transfer of the image to the card. Horizontal registration involves controlling the three independent systems so that the card and the desired printed image on the receptor web reach the nip of the applicator rollers simultaneously with the moveable applicator roller reaching its application position, thereby assuring that the printed image will be properly located horizontally on the card. Vertical registration involves controlling the relative vertical positions of the card and receptor web such that the printed image will be properly located vertically on the card.
Since card transport devices in a printer are typically driven with stepper motors, horizontal registration is usually accomplished in an open loop system, where the card is transported (or stepped) a predetermined number of steps where, on completion of the steps, the card is assumed to be properly located relative to the applicator rollers and the receptor web. The receptor web is also driven by an independent stepper motor, where on completion of a predetermined number of steps, the receptor web is assumed to be properly located relative to the card and the applicator rollers. The moveable applicator roller, as stated above, is also required to position itself prior to beginning the transfer of the image onto the card. The moveable applicator roller is also positioned using an independent stepper motor which drives the moveable applicator roller toward the other applicator roller a predetermined number of steps, such that upon completion of the steps, the moveable roller is assumed to be registered with the card and receptor web. Typically, the card and web are first driven into position and then stopped, waiting for the applicator roller to be driven into position so that the image transfer function can begin.
This method results in several problems however, such as card slippage due to having to stop and restart the stepper motor driving the card drive rollers. The receptor web is also liable to cut, slip or melt due to waiting for the registration to be completed before beginning the image transfer function. Further, an image transfer section such as this will experience problems when a component in any one of the systems requires replacement.
Further, vertical registration is difficult due to web tracking variances, since the receptor web may drift in the vertical direction. Therefore, either the card or the receptor web must be adjusted to match the vertical location of the image with the card. Further, since the plastic card and the image(s) printed on the receptor web are produced independent of each other, there is no correlation between the two mediums. A plastic card that is to be personalized with unique data, such as customer data, must be matched with the proper image that is printed on the receptor web. Failure to properly match the card with the correct printed image will result in, at best, card production errors, and at worst, the customer receiving the wrong card and/or the wrong personalized data.
Another problem faced by an image transfer section is the requirement to match the speed of the moving card with the speed of the applicator rollers and the speed of the receptor web. The independently driven systems must not overdrive the controlling critical element, in this case the applicator rollers, while the image transfer function is being performed, otherwise wrinkles, voids, "chatter", etc. may appear in the transferred image, or even worse receptor web breakage, card jams and the like may occur. Therefore, the card and the receptor web must be driven at the same speed as the applicator rollers. Typically, the card drive rollers and the web take-up roll are clutched, while the card and receptor web motions are controlled by the applicator rollers pinching the card and web, which allows the systems to be overdriven while avoiding the problems mentioned previously. However, since the applicator rollers are typically driven by a DC motor, the rotational speed thereof will vary due to the load (pressure) on the applicator rollers as they press together. The DC motor may also vary rotational speed due to the amount of current allowed to the motor under control of the microprocessor connected thereto.
What is needed then is a control system and method for an image transfer section that ensures horizontal and vertical registration, adjusts the speed of the web take-up roll and the card drive rollers to match the rotational speed of the applicator rollers, and ensures that the correct image is transferred onto the card.